1. Field of the Invention
The present invention relates to a semiconductor laser that outputs a laser beam utilizing self-coupled effect, an optical head, an optical disk apparatus and the manufacturing method of the semiconductor laser.
2. Description of the Related Art
As for an optical disk apparatus, the density and the capacity of an optical disk have been increased from a compact disc (CD) to a digital versatile disc (DVD), and further increase of the capacity is desired because of the improvement of the performance of a computer and the precision of a display.
The enhancement of the recording density of an optical disk is basically restricted by the diameter of a light spot formed on a recording medium. As a measure of acquiring an optical spot below the diffraction limit of a laser beam, a method of providing a small aperture on a light spot position of a transparent condensing medium and utilizing near-field light that leaks out from the small aperture is drawing attention. However, as efficiency for light utilization by this method is very small, the intensity of near-field light that leaks out is low and smaller recording mark than a recording mark (approximately 0.1 xcexcm) obtained by condensing by a conventional type lens has not been obtained yet.
As a measure of solving this problem, a method of recording and reproducing utilizing the self-coupled effect (SCOOP effect) of a semiconductor laser is proposed. That is, it is a method of forming a small aperture on a spot position on the output face of the semiconductor laser and using a laser beam leaked out from the small aperture for recording and reproduction, and particularly in reproduction a reflected light from a recording medium is re-incident into the resonator of the laser via the small aperture and is electrically or optically detected the modulation of the oscillated state of the laser. According to this method, as the sensitivity is high, reproduction is enabled even if re-incident light is weak.
Conventional type optical heads using this method are disclosed in xe2x80x9cTech. Dig. ISOM/ODS 1999, ThC-1 (1999) p. 352xe2x80x9d (hereinafter called Document I) by A. Partovi and in xe2x80x9cJpn. J. Appl. Phys. 38 (1999) Pt. 2, No. 11B, p. L1327xe2x80x9d (hereinafter called Document II) by S. Shinada, for example.
FIG. 14 shows the conventional type optical head disclosed in Document I. As for the optical head 1, an edge emitting semiconductor laser 2 is arranged at the rear end 11a of a flying slider 11. As for the edge emitting semiconductor laser 2, a high-reflective multilayer film 10a and a low-reflective multilayer film 10b respectively including a resonator having the oscillation wavelength of 980 nm are respectively arranged on the rear end face and the front end face of an oscillation area 8, and a metallic shade 4 having a small aperture 5 formed by etching using a focused ion beam (FIB) of Ga ions is arranged on the surface of the low-reflective multilayer film 10b. In such a configuration, recording and reproduction are performed by radiating a laser beam 6 of minute size leaked out from the small aperture 5 to a phase change-type recording medium 7a of an optical disk 7. In reproduction, information is reproduced by making reflected light from the recording medium 7a incident into the resonator of the semiconductor laser 2 via the small aperture 5 and inducing the self-coupled effect, that is, electrically or optically detecting the modulation by re-incident light of the semiconductor laser 2. The recording density can be enhanced by using the laser beam 6 made minute by the small aperture 5 for recording and reproduction.
FIG. 15 shows a conventional type semiconductor laser disclosed in Document II. The semiconductor laser 2 is a vertical cavity surface emitting laser 2 made of a semiconductor crystal of AlGaAs and oscillated at the wavelength of 850 nm, and high-reflective multilayer film 10a, a p-type AlAs layer 33, a p-type spacer layer 34, high-reflective multilayer film 10c having partial transmission and a phase compensation layer 35 are sequentially formed on a substrate 11 made of GaAs and a metallic shade 4 wherein a small aperture 6 is formed over an oscillation area 8 by etching using a focused ion beam is arranged on the output surface 3 of the semiconductor laser 2. The high-reflective multilayer film 10a and the high-reflective multilayer film 10c respectively of a resonator are respectively made up by alternately laminating a GaAs layer and an AlGaAs layer respectively having the thickness equivalent to a quarter wavelength. A mirror for the resonator on the output side is made up of the high-reflective multilayer film 10c and the metallic shade 4. Also, as reflection on the metallic shade 4 inverts the phase, the phase compensation layer 35 having the thickness in which optical path length is a quarter wavelength and made of AlGaAs is arranged between the high-reflective multilayer film 10c and the metallic shade 4 so that the reflection can be intensified. The recording density can be enhanced by using a laser beam 6 made minute by the small aperture 5 for recording and reproduction.
In the meantime, for a semiconductor laser having configuration different from that of the two conventional examples though it is a semiconductor laser that emits a laser beam utilizing self-coupled effect, there is the one disclosed on p. 27 of the 73rd Minute Optics Workshop Document (September, 1999) for example.
FIG. 18 show the semiconductor laser. The semiconductor laser 2 is a vertical cavity surface emitting laser and is provided with a beam-condensing body 61 in the shape of a pyramid, called as Total Reflection Tip, provided to the output surface of the laser 2 and made of semiconductor material, a conical central metallic body 66 provided to the end of the beam-condensing body 61, a minute coaxial body 65 configured by a carbon nanotube formed at the end of the central metallic body 66 and a metallic film 63 formed via a dielectric layer 62 around the beam-condensing body 61, the central metallic body 66 and the minute coaxial body 65. According to this configuration, as shown in FIG. 19, propagation light (in TEM00 mode) of minute size is obtained from the minute coaxial body 65.
However, according to the conventional type semiconductor laser shown in FIGS. 14 and 15, as an air gap is formed between the semiconductor laser and a recording medium and corresponds to the thickness of the metallic shade provided to the output surface of a laser beam, the output power is rapidly decreased more than quantity in inverse proportion to the area of the aperture even if the size of the small aperture is reduced to enhance the recording density, consequently the recording density cannot be enhanced.
That is, in the case of a simple aperture, when the diameter of the aperture is equal to or below a half of the wavelength, cutoff will be occurred as same as a wave guide of a microwave. The aperture becomes narrower, a laser beam which can pass the aperture will be decreased exponentially. Also, in that case, though a laser beam intervenes in the vicinity of an interface mainly as near-field light, the broadening width is approximately the aperture size. In case the aperture width is 100 nm, the depth and the aperture width are substantially equal as shown in FIG. 16A, the intensity of near-field light decreases in the direction of the normal line of the aperture exponentially as shown in FIG. 16B and a laser beam hardly reaches outside the surface 4a of the metallic shade 4. Hereby, when the aperture size is reduced as described above, power is rapidly decreased.
FIG. 17 shows relationship between the aperture size and the optical output power. As a recordable optical recording medium, a phase change-type recording medium mainly consisted of GeSbTe, for example, and a magneto-optic recording medium mainly made of FeTbCo, for example, are known and both require the optical power density of approximately 3xc3x97106 W/cm2 (20 mW in case the diameter of a light spot is 1 xcexcm) for recording. In the meantime, as shown in FIG. 17, in case the aperture size is 0.1 xcexcm and 0.05 xcexcm, the output power is respectively 0.1 mW and 0.01 mW, the optical power density is respectively 1.8xc3x97106 W/cm2 and 0.7xc3x97106 W/cm2 and is rapidly decreased to be a fraction of the power density required for recording, then the output becomes short to utilize.
FIG. 19 shows a problem of the conventional type vertical cavity surface emitting laser 2 shown in FIG. 18. The laser is configured so that most of output light from the vertical cavity surface emitting laser 2 is totally reflected twice on the slant face 61a of the beam-condensing body 61 in the shape of a pyramid as shown in FIG. 19 and is returned into the vertical cavity surface emitting laser 2, and the slant face 61a itself functions as a part of a reflector of the resonator on the output side of the laser 2. Therefore, the slant face 61a is formed so that the slope angle is 45 degrees. A dielectric layer 62 is provided outside the slant face, however, the refractive index of the dielectric layer 62 that can be formed by deposition and sputtering is at most 2.2, while as the refractive index of semiconductor for a laser is approximately 3.5 and the critical angle of incidence is approximately 38 degrees, incidence at an angle of 45 degrees surely meets a condition of total reflection and a laser beam is totally reflected on the slant face 61a of the beam-condensing body 61 of the semiconductor laser. Output light is to be emitted from a part the inclination of which at the vertex of the beam-condensing body 61 in the shape of a pyramid is reduced and which has curvature. However, the coaxial central metallic body 66 is formed in the part at the vertex, most of output light is reflected or absorbed on/into the central metallic body 66 and light hardly reaches the minute coaxial body 65. Even if light leaks from the slant face 61a of the beam-condensing body 61, the light is reflected in the direction of the laser by the metallic film 63 or is absorbed as well-known in a minute tapered probe, and a laser beam hardly reaches the minute coaxial body 65, either, and required intensity is not obtained.
The invention provides a semiconductor laser, an optical head, an optical disk apparatus and the manufacturing method of the semiconductor laser, in which the recording density of a recording medium can be enhanced by increasing the intensity of a laser beam output from a small aperture, and the miniaturization and the enhancement of a data transfer rate are enabled.
The invention provides a semiconductor laser having a metallic shade, which is a part of a resonator structure of the semiconductor laser, including a small aperture and located on a laser beam output side of the semiconductor laser. A material which can transmit a laser beam is buried in the small aperture.
According to the configuration, as the wavelength of a laser beam in the aperture becomes short in inverse proportion to the refractive index of the material that transmits a laser beam, the ratio of the aperture width to the wavelength becomes large, so a laser beam can pass through the aperture easier. Also, since near-field light is transmitted to the surface of the material buried in the small aperture in place of the original laser beam output surface, the gap between near-field light and a recording medium is reduced and near-field light can be efficiently incident on the recording medium.
The material, which can transmits a laser beam and is buried in the small aperture, can be arranged as a part of the semiconductor laser structure and a reflection-reducing film. As the semiconductor laser structure, there are a reflective film, a multilayer reflective film, an active layer, a cladding layer or a window material which has a band gap wider than the active layer and provided on the end face of the active layer, mainly, one or plural of these layers can be buried in the small aperture. Also, dielectric material such as SiO2 can be used for the reflection-reducing film.
The invention also provides a semiconductor laser in which a central metallic body is further provided at the center of the small aperture to enable the provision described above.
According to the configuration, a coaxial aperture is further formed by the small aperture and the central metallic body and propagation light is output from the axial aperture. Since propagation light is output from the surface of the material buried in the small aperture in place of the original laser beam output surface, the gap between the propagation light output surface and a recording medium is reduced and propagation light can be efficiently made incident on the recording medium.
The invention also provides an optical head having a semiconductor laser including a metallic shade, which is a part of a resonator structure of the semiconductor laser, including a small aperture and located on a laser beam output side of the semiconductor laser, and a flying slider that holds the semiconductor laser and flies over a optical disk are provided so as to enable the provision described above. A material which can transmit a laser beam is buried in the small aperture.
The invention also provides an optical disk apparatus characterized in that the optical disk apparatus is provided with an optical disk on which a recording medium is formed and an optical head provided with a semiconductor laser having a metallic shade, which is a part of a resonator structure of the semiconductor laser, including a small aperture and located on a laser beam output side of the semiconductor laser, and a flying slider that holds the semiconductor laser and flies over the optical disk are provided so as to enable the provision described above. A material which can transmit a laser beam is buried in the small aperture.
The invention also provides a manufacturing method of a semiconductor laser characterized in that semiconductor laser material having a laser beam output surface of the semiconductor laser is prepared, a concave portion is formed by etching and removing the periphery of an area corresponding to an opening on the laser beam output surface and the concave portion is coated with a metallic body by a photolithographic method or a focused ion beam so as to enable the provision described above.